The latest twist in childbirth prep involves jangling sequins and hip-shaking Middle Eastern music. As the growing popularity of belly dancing ripples across the U.S.--helping many a gymgoer wiggle off unwanted pounds--the ancient art form is also being practiced by moms-to-be to stay fit and ease their way through labor. From Georgia to California, dance instructors have started tailoring classes to help pregnant women with their flexibility, strength and breathing. "As soon as we feel pain, we tense up and hold our breath," says Stefanie Masters, who teaches two free classes a week in a maternity store in Brookfield, Wis. "Dancing...